The crystallization of sugar in the mills which produce it is divided into two processes which are complementary and which are carried out with very different equipment one from another. The first process is performed by evaporation of a sugar solution, which crystallizes over seed crystals until it forms a mass of crystallized material, and the second process is carried out by cooling the massecuite (cooked mass) composed of crystals and mother-liquor so that the sucrose of the mother-liquors is deposited on the crystals, until the former are exhausted.
In the first process mentioned, in which crystallization is carried out by evaporation, the apparatus used at present, known as vacuum pans, present a large number of drawbacks from the point of view of yield and operation.
One such drawback is that the mass being processed is concentrated by evaporation in the vacuum pans until it reaches the zone of super-saturation known as the metastable zone, in which the quantity of sucrose contained in the solution is greater than that contained in a saturated solution, and that is the moment at which seed crystals are seeded to accomplish the crystallization of the sugar solution. Nevertheless, the meta-stable zone is limited at the top by a solution of a concentration in which new nuclei of crystals form spontaneously, and at the bottom by a solution which first reaches saturation and then becomes unsaturated with the crystals dissolving again into the solution. Consequently within the meta-stable zone there prevail different speeds of crystallization due to the different concentrations appearing at the same temperature; this situation causes the crystals to develop differently, if the mass is not maintained uniformly in process, and the product contained will be made up of crystals of different sizes, which is undesirable.
Another disadvantage is that when there is an excess of mother liquors, upon increasing the concentration of the mass in process (i.e. the mass undergoing crystallization) by evaporation there is the danger that undesirable new crystals known as false grains will form. Likewise, in the vacuum pans the mass cannot be processed in high concentrations because it would tend to clog; therefore the processing is done in low concentrations, and consequently the speed of crystallization is low requiring an extended processing time with a greater expenditure of energy.
Finally, because of the shape of the vacuum pans and because movement of the mass being processed in them is generally effected only by boiling, the concentration of sucrose in the mass at discharge is not optimum, causing low recovery and thus requiring a greater recirculation of mother liquors.
On the other hand, the process of crystallization by cooling also has several drawbacks in comparison with the process of crystallization by evaporation. Thus to carry out the process of exhaustion of the mother liquors of a cooked mass (massecuite) in which the quantity of sucrose which crystallizes is on the order of 10%, from twice to six times as much time is required as in the process of crystallization by evaporation, in which a large quantity of sucrose is deposited by crystallization upon the crystals in less time. Furthermore, when there is an excess of mother liquors in a cooked mass, upon cooling it to bring about crystallization, thereby to exhaust the said mother liquors, new crystals of false grains are produced.
In addition, the equipment in which this process of crystallization by cooling is carried out is very large, hence expensive, difficult for maintenance and of low efficiency.